Hydroalkylation of alkenes with 1,3-diketones via gold(III) or silver(I) catalysis: divergent mechanistic pathways revealed by a DFT-based investigation

Density functional theory calculations were used to investigate the mechanisms of established hydroalkylation reactions of styrenes with 1,3-diketones that are promoted by either AuCl₃/AgOTf or AgOTf catalyst systems. In the former case, our studies led us to propose an original mechanism that is initiated by the generation of highly electrophilic Au(OTf)₃, which then coordinates the enol tautomer of the 1,3-diketone substrate. The ensuing highly Brønsted acidic π-complex serves to protonate the styrene to generate a relatively low-energy benzylic carbocation. Notably, this suggests that this benzylic carbocation represents the true catalytic species in the reaction, and thus, the role of the gold complex is solely to generate this active catalyst. AuCl₃ alone does not serve as a good initiator for this process because it is not electrophilic enough to generate the relatively low-energy benzylic carbocation. Our investigation of the hydroalkylation facilitated by the slightly electron-deficient AgOTf catalyst revealed that an alternative mechanism predominates. Specifically, it is more likely that the reaction proceeds via a demetallation process directly mediated by the silver catalyst. We found a clear trend indicating that the electron deficiency of the metal center dictates which of these two mechanistic scenarios occurs. This article discusses these two mechanistic pathways in detail, providing key information for the experimental development of hydroalkylation processes.